1. Field of the Invention
The present invention relates to a stainless steel, for a brake disc, excellent in resistance to temper softening and, more specifically, to a martensitic stainless steel having excellent resistance to softening resulting from heat generation caused by braking, as well as satisfying the hardness, hardenability and rusting resistance required of a material for a brake disc of a two-wheeled vehicle, a snowmobile or the like.
2. Description of the Related Art
A material for a brake disc of a two-wheeled vehicle, etc., is required to have properties such as abrasion resistance, rusting resistance and toughness and a JIS SUS410 type martensitic stainless steel has been mainly used for such applications. Although abrasion resistance is generally improved as hardness is increased, so-called brake noise is generated, by the contact between brake and pads, when the hardness is too high. The hardness of a brake disc is, therefore, controlled to within a prescribed range.
In order to stably control the hardness to within a prescribed range after quenching heat treatment and to ensure sufficient toughness and rusting resistance, the chemical compositions of steels with regulated amounts of Mn, Cu, etc., as well as C+N, are disclosed in Japanese Unexamined Patent Publications No. 57-198249, No. 59-70748 and No. 10-152760 and Japanese Published Patent No. 2-7390. Namely, these represent methods to obtain desired hardness after quenching by controlling the contents of C and N, preceded by sufficiently ensuring the austenite phase at a high temperature by controlling the contents of Mn and Cu in steel. Owing to these chemical compositions, it has been possible to provide materials for brake discs for two-wheeled vehicles satisfying abrasion resistance, rusting resistance and toughness.
In the meantime, the performance of recent two-wheeled vehicles calls for higher braking force at higher speed and it is required that the material properties, especially hardness, of a brake disc do not deteriorate when the disc is heated to a temperature exceeding 500xc2x0 C. during braking. Since the above-mentioned steels have a matrix of martensitic phase, however, they undergo temper softening when heated to a temperature exceeding 500xc2x0 C. And thus the development of a material for a brake disc satisfying the above requirements has been sought.
An object of the present invention is to provide a stainless steel for a brake disc less susceptible to deterioration of material properties, especially the deterioration of hardness, when heated to a temperature exceeding 500xc2x0 C. during braking, while maintaining the abrasion resistance, rusting resistance and toughness of a conventional steel.
In order to attain the above-mentioned object, the present inventors investigated and experimentally studied chemical compositions and additive elements which do not impair the original properties required of a brake disc and do not significantly increase the cost. As a result, the present inventors found that the temper softening temperature for lowering the hardness to below 30 HRC (Rockwell hardness C scale) can be increased by 30xc2x0 to 100xc2x0 C. or more by restricting the range of chemical composition so as to maintain the hardness after quenching, which relates to the abrasion resistance, in the range of 30 to 40 HRC, preferably 32 to 38 HRC and so as not to deteriorate rusting resistance and toughness, and then by adding an appropriate amount of Nb which suppresses temper softening.
The present invention was completed based on the above-described knowledge and the gist of the present invention is as follows:
(1) A stainless steel for a brake disc excellent in resistance to temper softening, characterized in that: said steel contains, in terms of wt %,
C: 0.01 to 0.1%,
N: 0.03% or less,
C+N: 0.04 to 0.1%,
Si: 1% or less,
Mn: 2% or less,
Ni: less than 0.5%,
Cr: 10 to 15%, and
Nb: 0.02 to 0.5%,
with the balance Fe and unavoidable impurities; wherein the hardness after quenching heat treatment is in the range of 30 to 40 HRC; and the temper softening temperature for lowering the hardness to below 30 HRC is at least 530xc2x0 C.
(2) A stainless steel for a brake disc excellent in resistance to temper softening according to item (1), characterized by further containing, in terms of wt %, at least one of the following elements:
Cu: 0.1 to 2%, or
Mo: 0.1 to 1%.
(3) A stainless steel for a brake disc excellent in resistance to temper softening according to item (1) or (2), characterized by further containing, in terms of wt %, at least one of the following elements:
Ti: 0.01 to 0.5%,
V: 0.01 to 0.5%, or
B: 0.0005 to 0.01%.
(4) A stainless steel for a brake disc excellent in resistance to temper softening according to any one of items (1) to (3), characterized in that the value of xcex3p calculated from the steel chemical composition as expressed below is at least 70%.
xe2x80x83xcex3p=420xc3x97[% C]+470xc3x97[% N]+23xc3x97[% Ni]+9xc3x97[% Cu]+7xc3x97[% Mn]xe2x88x9211.5xc3x97[% Cr]xe2x88x9211.5xc3x97[% Si]xe2x88x9212xc3x97[% Mo]xe2x88x9247xc3x97[% Nb]xe2x88x9252xc3x97[% Al]xe2x88x9249xc3x97[% Ti]xe2x88x9223xc3x97[% V]xe2x88x92500xc3x97[% B]+189